1. Field of the Invention
This invention relates to fuel injection systems for vehicle engines and more particularly to a closed loop system for reducing noxious components in exhaust emitted from the engine.
2. Description of the Prior Art
During the last ten years or so, it has become necessary to design internal combustion engines emitting low levels of the pollutants carbon monoxide, hydrocarbons, and oxides of nitrogen. Many of the techniques devised for this purpose are successful only if very accurate control is maintained over the air/fuel ratio (A/F) of the mixture entering the combustion cylinders. The air/fuel ratio should be controlled to within about one percent to enable the three way catalyst that is generally employed in such systems to function effectively.
One of the most promising methods for obtaining close control of A/F is based on a device sensing oxygen content of the exhaust gas. The output signal of such a device is a strong function of the air/fuel ratio, especially when such air/fuel ratio approaches stoichiometry. By using the output signal of the oxygen sensor to make appropriate corrections in the fuel flow rate, a closed loop control system is achieved.
In principle, closed loop systems are self-correcting, highly accurate, and never require recalibration. In practice, however, closed loop control systems are subject to the occurrence of instabilities, leading to large amplitude fluctuations and loss of control. One of the possible causes for such instabilities is the existence of a transportation delay or "dead time" somewhere in the controlled system. This transportation delay consists of the time interval between formation of the mixture, and arrival of the burnt mixture at the exhaust oxygen sensor.
Closed loop systems have heretofore endeavored to eliminate transportation delay instability by reducing the total gain of the loop. The loop gain reduction is preferably set so that the time for response to a disturbance is about equal to the dead time. The latter is not constant, but varies with engine speed. Hence, the loop gain must be long enough to permit the system to remain stable under engine idle conditions, wherein dead time is longest. Adjusting loop gain so that the time for response to a disturbance is substantially the same as dead time under idle conditions produces a response time that is too long for optimum air/fuel ratio control at higher engine speeds.